1. Field of the Invention
The present invention relates broadly to inter-axle differential assemblies and, more particularly, to a dedicated lubrication pump for an inter-axle differential assembly.
2. Discussion of Related Art
Motor vehicles with solidly connected multiple drive axles are commonly equipped with an inter-axle differential assembly, commonly arranged in a vehicular transmission transfer case or tandem axle power divider to allow torque balance between the drive axles during the vehicle cornering, to compensate for tire size differences, etc., i.e. when there is any physical requirement for speed difference between the drive axles. The inter-axle differential assemblies are widely employed for tandem drive axles of heavy-duty trucks for on- and off-road service as a power divider.
These motor vehicles are, on occasion, driven in situations where there may be unequal traction conditions between the tires of the different drive axles. If the traction condition at any tire falls below that required for sufficient traction effort, high-speed inter-axle differential conditions may occur. These high-speed differential conditions may be potentially severely damaging to critical differential assembly components, such as shaft bearing surfaces as well as rolling contact surfaces of the differential assembly, due to lack of lubrication. In such drive axles it is common to have a supply of lubricant in a transfer case or axle housing and to provide positive lubricant pressure to the input and output shaft journals and the inter-axle differential that are disposed above the level of lubricant in the housing to prevent damaging the differential gear components during these high speed differential conditions. However, current lubrication pumps for differential assemblies are driven continuously while the vehicle is in motion, although lubrication supply is only needed during occasional conditions of relatively high-speed levels of differential action as it is well known to those skilled in the art. The continuously driven lubrication pump operates and consumes engine power irrespective of the amount of lubrication needed by the shaft journals and other components of the inter-axle differential, thus causing unnecessary parasitic losses in a vehicle power transmission and increasing fuel consumption.
Because differential rotation may occur in either rotational direction, lubrication pumps must be reversible. In other words, the pump must output lubricant in the same direction regardless of the direction of relative rotation. Some conventional pumps include an eccentric ring disposed about inner and outer rotational members (e.g., the rotor and impeller) of the pump that includes an arcuate groove. Rotation of the ring relative to the outer rotational member switches the eccentricity of the rotational members. Other conventional pumps include a porting plate disposed at one end of the inner and outer rotational elements. The porting plate likewise has an arcuate groove and can rotate relative to the inner and outer rotational members. Rotation of the plate maintains the eccentricity of the rotational members, but swaps the input and output ports of the pump. These conventional pumps are disadvantageous, however. First, the use of a reversing ring or porting plate results in a larger pump and compromises pump displacement. Second, the use of a reversing ring or porting plate requires that the inlet and outlet ports for the pump be symmetrical which reduces the efficiency of the pump.
The inventors herein have recognized a need for a lubrication pump for an inter-axle differential assembly that will minimize and/or eliminate one or more of the above-identified deficiencies.
The present invention alleviates the drawbacks of the prior art. The present invention provides an inter-axle differential assembly having a dedicated lubrication pump. The lubrication pump is drivingly coupled to two differentially rotating members of the differential assembly, and, thus, supplies lubricant only when differential action occurs. The hydraulic pump provides volumetric flow of lubricant that varies in direct proportion to the relative (or differential) rotational speed of the rotating members.
In accordance with the preferred embodiment of the present invention, the inter-axle differential assembly comprises an input shaft, an output shaft arranged coaxially with respect to each other, a differential gearing and the dedicated lubrication pump disposed between the input and output shafts. The lubrication pump is provided solely for the purpose of lubricating the shaft journals and the inter-axle differential gearing, and only when needed, i.e. the pump generates lubricant flow only during the differential action between the input shaft and the output shaft, and at a flow rate in proportion to the speed differential.
In accordance with the preferred embodiment of the present invention, the pump is of the gerotor type, and the differential is of the bevel gear type. However, other types of pumps, such as gear or vane type pumps, are within the scope of the present invention, as well as other types of differentials, such as the spur gear type. The lubrication pump includes a rotor driven by the input shaft, and a housing coupled to a side gear drivingly connected to the output shaft. Alternatively, the pump housing is coupled directly to the output shaft. The housing defines first and second ports and further defines an arcuate groove in a radially outer surface. A pin extends radially inwardly from the side gear and is received in the groove. The groove cooperates with the pin to allow rotation of the entire pump and thus enables the pump to be reversible. In response to rotation of the output shaft in a first direction relative to the input shaft, the first port assumes an inlet position and the second port assumes an outlet position. In response to rotation of the output shaft in a second direction relative to the input shaft, the first port assumes the outlet position and the second portion assumes the inlet position. An oil flow generated by the lubrication pump is supplied to the shaft journals and the inter-axle differential gearing through a gallery communicating with passages in the input and output shafts which supply lubricant to the journals for these shafts and to the inter-axle differential assembly.
Therefore, the inter-axle differential assembly in accordance with the present invention includes the dedicated lubrication pump, compactly disposed between the input and output shafts, that lubricates the differential assembly components only when needed, thus providing better efficiency and lower fuel consumption.